The invention is related to the field of optical resonators, and in particular to a device structure of patterned nonreciprocal optical resonators for integrated optical isolator applications.
An isolator is a device that allows polarized light to pass through in one direction, but not in the opposite direction (like a one-way valve or diode). Optical isolators are indispensible devices in optical communication systems which prevent laser degradation, optical crosstalk and signal instability due to back scattering. Conventional bulk optical isolators used in optical communication at around 1550 nm wavelength are based on the Faraday effect, in which a magnetooptical single crystal of (Bi,Y)3Fe5O12 (garnet) is placed in the path of the light providing this effect, together with a polarizer and analyzer with their propagation angles at 45° to each other, in front and behind the crystal respectively. The forward light is linearly polarized through the first polarizer. Then the polarization direction is rotated 45° after passing through the magnetized garnet crystal and the light can pass through the analyzer. In contrast, the reflected light has its polarization direction rotated 45° due to the non-reciprocity of the garnet crystal and is blocked by the initial polarizer.
With the development of silicon photonics and increasing of the scale of integration photonic circuits, integration of optical isolators is becoming increasingly urgent. Currently, monolithic integration of an optical isolator on a semiconductor platform remains challenging for integrated photonic systems. Optical isolators based on Mach-Zehnder structure have been proposed and demonstrated on a garnet substrate platform. However such devices usually require larger footprint compared with the Faraday isolator counterpart owing to the weaker magneto-optical nonreciprocal phase shift (NRPS) effect in a Mach-Zehnder configuration. In order to reduce the device footprint and enable the integration of optical isolators on a semiconductor platform, devices based on optical resonance have been proposed. Using resonance structures such as ring resonators or photonic crystals, the footprint of optical isolators were expected to significantly reduce from millimeter to micron meter level.
However, all previously proposed device structures either require patterning and etching of the magneto-optical materials, or engineering the magnetic domain structures of the magneto-optical materials, or engineering a non-homogenous applied magnetic field on the resonator structure, which are highly fabrication unfriendly. Until now the functionality of these devices has not been demonstrated experimentally due to fabrication difficulties. Therefore, it is highly desired that a fabrication friendly, monolithically integrated device structure which uses homogenous magnetic field and magneto-optical material can be developed for optical isolation applications.